The invention relates to surface combustion infrared burners. In these burners the radiating surface of the burner is a body of porous material. A fuel/oxidant gas (e.g., fuel/air) mixture passes through the porous material and burns at the surface from which it emerges. The surface is heated by the flame and becomes the radiating surface of the burner. The porous material according to this invention is a multiple layer porous material. Such burners are disclosed, for example, in U.S. Pat. Nos. 1,830,826; 3,912,443; 3,216,478; 3,738,793; 3,947,233 and 4,643,667.
Surface combustion burners have many uses including, for example, use as space heaters, paint dryers, industrial process dryers for paper, plastics and ceramics, defrosting, food processing including deep fat fryers and broilers and deinfestation of grains.
Any warm body produces infrared energy. The higher the temperature of the body, the shorter the wavelength. Infrared energy is difficult to produce simply from a flame. Only about 10 to 20 percent of flame energy is infrared radiation. Solids in contact with flames can be excellent sources of infrared radiation and by controlling the temperature of the surface of the solid the wavelengths over which the radiation is distributed can be controlled.
It is known in the art that there are a number of surface combustion burners in which the porous material is porous refractory (ceramic), perforated ceramic, honeycomb ceramic, ceramic and metal fibers, drilled stainless steel, or even a metallic screen. A common expedient is to utilize a metal screen spaced from the combustion surface or a layer of increased pore size to hold the flame. The screen requires support from the burner plate surface and the supports reduce the effective emitter surface area by shielding it. This reduces the infrared output and can cause overheating of the burner plate. Additionally, the screen is a source of failure due to oxidation and slumping. Typically, the limited temperature resistance of the screen limits the temperature of the burner.
In any surface combustion burner the temperature of the inlet side of the burner plate must not become too hot because of the possibility of a backfire, that is, flame will occur on the wrong side of the porous material. The density and thickness of the plate of porous material as well as the rate of flow of gas through the plate and the temperature at the radiating side of the plate will affect the temperature on the inlet side of the plate. It is desirable that the flame be localized behind or near the radiating surface of a surface combustion burner since the essential purpose is to heat the surface by contact with the hot gases constituting the flame. The efficiency of the burner can be improved if the flame is maintained within the porous material and near the exit surface.
The flame in a surface combustion burner can be located within the porous material and near the exit surface by utilizing a burner plate having layers of different pore sizes. A fine pore base provides the initial combustion surface and insulation protecting against backfire while the flame is stabilized within a larger pore layer at the radiating surface. Increased efficiency and flame stability results from the use of the multiple layer burner body. Improved efficiency results in fuel savings. Improved flame stability allows for more versatile use. Also, the temperature range may be greater and the ratio of fuel to oxidant gas may extend over a larger range.